1. Technical Field
The present invention relates to industrial trucks, and particularly to fork lift trucks which use hydraulically operated linear actuators to control movement of the forks of the truck.
2. Description of the Prior Art
A typical prior art fork lift truck has load forks mounted on a carriage. The carriage is movable vertically on a mast assembly to raise and lower the forks. The mast assembly including the carriage can be tilted forward and back about a transversely extending tilt axis. In some fork lift trucks, the carriage with the forks can also be extended and retracted, and shifted from side to side. Each of these functions is controlled by a separate hydraulic cylinder assembly.
The flow of fluid into and out of the hydraulic cylinder assembly is controlled one or more of valves located in line between a pump and a tank or return. Double-acting hydraulic cylinder assemblies are most often used. Each double-acting hydraulic cylinder assembly has a piston slidably disposed in a cylinder. An actuator rod extends axially from the piston through one end of the cylinder and is connected to the portion of the carriage or mast to be moved. The piston divides the cylinder into a rod end chamber and a piston end chamber.
A four-way three-position directional control valve is normally used to control the operation of the double-acting hydraulic cylinder assembly. In a first position, the valve directs fluid under pressure to the rod end of the cylinder and allows displaced fluid to flow from the piston end of the cylinder through the valve to return. The actuator rod moves toward the piston end of the cylinder. In a second position, the valve directs fluid under pressure to the piston end of the cylinder and allows displaced fluid to flow from the rod end of the cylinder through the valve to return. The actuator rod moves toward the rod end of the cylinder. In a third position, no fluid can flow through the valve, and the hydraulic cylinder assembly is not actuated to move in either direction.
Although it works adequately to control the hydraulic cylinder assembly, a four-way three-position directional control valve is relatively large and expensive. Using a separate four-way three-position valve to control each of the four above-mentioned functions in a lift truck entails a substantial expense and uses a substantial amount of space. It would be desirable to provide a different manner of controlling the actuation of the hydraulic cylinder assemblies in a fork lift truck which would require less space and would also cost less.